Remove common part of path from VS filters when generating workspaces

Currently when generating filters for VS workspaces, a filter is added for each directory along a file's absolute path, which is annoying when all your source files are quite close together but far away from the root - you end up having to expand a series of filters each with just one child.

This is a simple change to find how much of the path all solution items share, then remove that much of the path from the filters.

It works well for me, best check it yourself before merging (if you choose to merge) since I'm not sure it handles ignoring jam files properly, and have left a comment to that effect.